1. Technical Field
The present invention combines a fluidized bed apparatus with electrochemical deposition of coatings on metal, alloy, polymer, and ceramic materials, thereby completely and uniformly coating a wide variety of materials. More particularly, the invention provides a method and apparatus for coating particulate substrates resulting in coated particles useful in applications requiring enhanced thermal properties and in applications where coated particulates provide advantages in consolidation.
2. Description of Related Art
Several industrially accepted techniques of coating materials with metals or alloys exist. These techniques include (1) immersion deposition, wherein coating occurs by immersion of an object in an electrolyte solution without application of an external current, and where the coating metal or alloy is more noble than the material being coated; (2) electrolytic fluidized bed deposition, wherein a bed of material to be coated is itself fluidized by pumping an electrolyte solution through the bed; (3) autocatalytic deposition, which requires a catalyst and a reducing agent; (4) dry fluidized bed deposition, wherein coating is accomplished by chemical or physical vapor deposition; and (5) mechanical plating, wherein both the metal coating and the material to be coated are powders, and physical means are used to coat the powders by impacting one metal onto another.
All of these techniques have significant disadvantages for coating particulate substrate materials, particularly in the areas of yield, flexibility, cost, process control, and coating thickness uniformity. For example, immersion deposition powder coating is restricted to specific situations where the coating metal or alloy is more noble than the substrate being coated. In addition, the composition of the electrolyte solution changes significantly with time so that the morphology of the coating also changes with time. Furthermore, with immersion deposition, certain sites remain uncoated on the surface of the substrate and impurities are often undesirably incorporated into the coating.
Electrolytic fluidized bed deposition as commonly practiced operates by pumping an electrolyte through a bed of particles or by passing the entire bed of particles across a cathode plate. This pumping process results in a solution of particles in which the particles are not in intimate contact and are undesirably dispersed. In other words, a less than ideal density of the fluidized bed is created as there is diminished particle to particle contact, as well as diminished particle to cathode contact. This results in a significant disadvantage in using this technique because a very high number of contacts are being continually made and broken, producing non-uniform distribution of charge. This latter phenomenon also results in large variations of potential distribution so that, in some parts of the bed, anodic reactions occur while in other parts cathodic reactions of differing potential occur, resulting in differing morphology or alloy compositions being deposited. Another disadvantage is that many metal/alloy combinations cannot be uniformly coated and control over the process is not precise.
Autocatalytic deposition is an aqueous electrochemical process in which the electrons required for the reduction reaction are provided by a reactant which itself is oxidized in the solution employed. No external current flows into the system and often the oxidized species is incorporated into the deposit. Autocatalytic deposition requires that the surface of the substrate be treated with a suitable catalyst such as platinum, palladium or tin and that a reducing agent such as formaldehyde, sodium hypophosphite, or sodium borohydride be present within the electrolyte. Examples of autocatalytic deposition are systems involving the metal-metalloids, such as nickel phosphorus, cobalt phosphorus, cobalt tungsten-phosphorus, or carbon containing metals such as copper, silver, and gold. In the metal-metalloid system, a sodium hypophosphite electrolyte is typical while in the copper system, a formaldehyde electrolyte is typical.
Autocatalytic deposition, however, has several disadvantages. First, most autocatalytic deposition processes result in the formation of an alloy coating around the particle being plated (hydrazine-containing systems are a notable exception). This alloy coating contains reaction products usually resulting from the reducing agent. For example, in the case of sodium hypophosphite, phosphorus is commonly incorporated into the coating to yield an alloy in which undesirable inclusions of phosphorus are present. In the case of copper, an undesirable carbon-copper alloy results from undesirable inclusions of carbon when a formaldehyde electrolyte reducing agent is used. These inclusions compromise the purity and uniformity of the resulting alloy coating as well as the properties of such materials.
The incorporation of almost any reaction product as an undesirable inclusion in the coating will generally have some effect on all of the properties of the coated particles. These undesirable inclusions have an adverse effect on the properties, particularly thermal properties, of the coated particles, particularly carbon inclusions in copper coatings.
The undesirable inclusions increase the electrical resistivity of the coated material, decrease the density, decrease the ductility, affect the melting point, and, most dramatically, decrease the thermal conductivity. Moreover, upon thermal treatment, the inclusions typically can cause undesirable phase changes. In the case of carbon impurities in copper coatings, the carbon-containing organic molecules expand and cause a significant density change. In the case of nickel (or cobalt) with phosphorus impurities, thermal treatment results in the formation of intermetallics such as Ni.sub.3 P.
Furthermore, autocatalytic deposition requires that a catalyst such as platinum, palladium or tin be adsorbed on the surface of the substrate. The presence of this catalyst further compromises the uniformity and purity of the final coated material. Also, the use of catalytic agents, particularly on particles with very high surface area to weight ratios, tends to make the electrolyte quite unstable in the sense that the entire system can spontaneously decompose. Furthermore, in the case of platinum and palladium, the catalytic agent is expensive, particularly when adsorbed on substrates with high surface areas.
In addition, autocatalytic deposition is limited in terms of the different metals which can be used for coating. Autocatalytic deposition is possible for some of the more common metals used as coatings such as Cu, Ni, Co, Ag, Au, Sn, Zn, Pd, Ru, and Fe. However, in most cases undesirable inclusions of carbon, phosphorus, or boron impurities remain in the coating.
Finally, not only are autocatalytic deposition processes typically quite expensive and inconvenient to use because of the chemicals required (Pt, Pd, etc.), but also because of the high temperatures used, the required degree of control, and the required ventilation.
The most significant disadvantage of chemical vapor deposition (CVD) of metals onto dry fluidized beds is the limitation of suitable substrate/coating combinations. For example, low melting point substrates cannot be used given the elevated temperatures involved in many CVD processes. Another disadvantage is that gas phase reactions occur which result in undesirable inclusions being incorporated into the coating, compromising its uniformity. The coating process of this technique is also difficult to control and often involves relatively high cost.
Mechanical plating of powders, while usually a low cost process, has the disadvantage of being restricted for applications using larger powder sizes. Furthermore, this technique requires the presence of an inert hard species which kinetically impacts the coating media as it is pressed into or onto the powder being coated. This necessarily results in the mechanical deformation of the powders and control of the process is difficult.
Physical blending or mixing of particles also poses significant disadvantages. If, for example, powders of copper and tungsten were simply physically mixed prior to compaction, the blending would be far from uniform. The reason for this is the different densities of copper and tungsten. In addition to density, differences in aspect ratio, roughness, or size will often result in segregation during blending. In fact, almost any discernible difference between powder types will result in segregation and some non-uniformity during the blending process. As a result of the non-uniformity of the composition and the concomitant lack of uniformity of properties, these composite powders frequently do not meet performance specifications. This failure to meet specifications within a high degree of accuracy is extremely important when large numbers of articles are manufactured and each one is critical to the performance of a larger device.
Thus, there is a need for a process to uniformly and completely coat particulate substrates with metals and alloys such that the final coated product does not contain undesirable inclusions which detrimentally affect its purity, its structural integrity at high temperatures, or its physical properties, including resistivity, density, ductility, and particularly its thermal diffusivity. It is also desirable for the coating thickness to be accurately controlled and for the coating process to be useful in applications using a wide variety of substrate materials.
In addition to the formation of coated particles with enhanced thermal properties, the present invention may find application in the area of dental restoration. Dental practitioners commonly employ silver-tin alloys and related intermetallic compounds as dental amalgam preparations for use in dental restorations such as fillings and prostheses. The typical amalgamation reaction uses elemental mercury as the liquid sintering agent in combination with the silver-tin alloy. However, certain mercury phases which constitute a network holding the amalgam together are susceptible to corrosion with concomitant release of elemental mercury and mercury-containing compounds.
Thus, in terms of dental restoration techniques, the accepted method of using mercury (and copper) amalgams presents two main disadvantages. First, the potential for mercury leaking from improperly formed amalgams and through corrosion presents mercury poisoning hazards to those exposed to such amalgams, such as patients treated with amalgams in dental applications. Second, the accumulation of toxic metals, namely mercury, in local sewage systems purportedly from dental office waste disposal of dental amalgams has raised public health concerns. Because of the possibility of mercury poisoning of the patient and the difficulties in disposal of mercury-containing amalgams, it is desirable to form less toxic alternatives to mercury amalgams for dental restoration applications. Health concerns mandate and, in the United States, Congress may require alternate mercury-free dental restoration materials.
Electrolytic fluidized bed reactors have been used to remove trace metals from industrial process waste streams. Such reactors have primarily been applied for removal of trace amounts of copper. In these fluidized bed systems, the bed of powders onto which the trace metals deposit is fluidized by flowing the electrolyte solution through the bed of powders. Often the cathode compartment is separated from the anode compartment by a membrane, such that separate catholyte and anolyte solutions exist. It is the catholyte solution which is being purified. While these systems may be effective in removing trace amounts of copper from waste streams, such systems have not found widespread applicability for use in coating a variety of substrate materials with uniform coatings.